How do atoms make compounds? Typically they join together in such a way that they lose their identities as elements and adopt a new identity as a compound. These joins are called chemical bonds. But how do atoms join together? Ultimately, it all comes down to electrons. Before we discuss how electrons interact, we need to introduce a tool to simply illustrate electrons in an atom.

The Lewis Structure of a molecule shows how the valence electrons are arranged among the atoms of the molecule. Lewis electron dot diagrams use dots to represent valence electrons around an atomic symbol. Lewis electron dot diagrams for ions have less (for cations) or more (for anions) dots than the corresponding atom. From experiment, chemists have learned that when a stable compound forms, the atoms usually have a noble gas electron configuration or eight valence electrons.

The tendency to form species that have eight electrons in the valence shell is called the octet rule. The attraction of oppositely charged ions caused by electron transfer is called an ionic bond. The strength of ionic bonding depends on the magnitude of the charges and the sizes of the ions.

Covalent bonds are formed when atoms share electrons. Lewis electron dot diagrams can be drawn to illustrate covalent bond formation. Double bonds or triple bonds between atoms may be necessary to properly illustrate the bonding in some molecules.

Lewis dot symbols provide a simple rationalization of why elements form compounds with the observed stoichiometries. A plot of the overall energy of a covalent bond as a function of internuclear distance is identical to a plot of an ionic pair because both result from attractive and repulsive forces between charged entities. In Lewis electron structures, we encounter bonding pairs, which are shared by two atoms, and lone pairs, which are not shared between atoms.

Covalent bonds can be nonpolar or polar, depending on the electronegativities of the atoms involved. Covalent bonds can be broken if energy is added to a molecule. The formation of covalent bonds is accompanied by energy given off. Covalent bond energies can be used to estimate the enthalpy changes of chemical reactions.

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